...Should i just cancel the appointment or hope that they might have some of the correct ones i need in stock?

I bought my wheels at Discount Tire, and they supplied the lugnuts. I would think that any tire shop that sells wheels would carry the lugnuts you'll need.

Or just buy a “Gorilla Wheel Installation Kit” (pictured below) that includes 20 small diameter, splined chrome lugnuts (12mm x 1.5) and the special socket for installing them. This is what I got from Discount Tire. I think they only charged me $15 for these.